If plugins have been disabled in your install of Lightroom because of a bad serial number, they will remain disabled until revalidated with a legal install.

If you think you bought your serial number (e.g. for a deep discount on eBay), you likely gave your money to crooks for a pirated number that cost them nothing and got you nothing. Some tips.

To correct your situation: acquire a legal copy of Lightroom from a reputable source, such as Amazon.com (upgrade or full) or immediate download at Adobe, or the Mac App Store, of perhaps subscribe to one of the Creative Cloud packages.

By all means, stay away from eBay... buying from shady sources there is what gets a lot of people into this situation in the first place.

How to revalidate depends on what kind of Lightroom install you have... there are two options:

Creative Cloud install (Lr5.5 or later) or a Mac App Store install:

You're done... the plugins should revalidate themselves the first time you launch your new Lightroom.

(You may, however, have to visit them in Lightroom's Plugin Manager to re-enable them; see below.)
Stand-alone purchase yielding your own legal Lightroom serial number:

Revalidating with your new Lightroom serial number requires a few steps:
1. Tell Lightroom about your new serial number.
  
  Invoke Help > Lightroom Registration, then on the right side of the dialog that pops up, click on “License” and replace the problematic number with your new number.
2. Restart Lightroom.
  
  On Windows, I've had reports that you may need to actually log out of your account and log back in again for the new serial number to fully take hold.
3. Email your Lightroom serial number to me with a request for a revalidation code.
  
  Prior to Lightroom 5.4 you could just copy it from the Help > System Info dialog, but from Lr5.4 it no longer shows. My System Info plugin can show your serial number; shift-click on the "keep private" note to bring up a dialog where you can copy the serial number to your clipboard, which you can then paste into an email.
  
  Do not mail a screenshot... I need to be able to cut-n-paste the serial from your message.
  
  Also, do not just copy the serial number from the box or from your receipt... sometimes Lightroom changes the serial on the fly.
4. I process revalidation requests in batches, every few days. I'll reply with a single line of text that you should copy-and-paste into the “revalidation code” box that appears in the top section of the Plugin Manager.
  
  Note: I won't respond if you mail another pirated number.
5. Restart Lightroom.

After either method outlined above, all plugins should now be working again, but when locked earlier they likely became disabled in the Plugin Manager; click on the “Enable” button in the middle-right section of the dialog, for each plugin.

A side effect of changing your Lightroom install to a legal one is that previous plugin registrations, if any, are lost. Plugin registration is not required, but if you'd like to have them registered, you'll need a new registration code (which can be created with a simple one-cent transaction if you like; see here).

FAQs...

Why do you care about the Lightroom serial number... what business is it of yours?
The registration system for my plugins is built on the normally-valid assumption that a Lightroom serial number is unique to each user, so it breaks when serial numbers are shared.

But anyway, it should be clear from my blog that I really like Lightroom, and I want it to flourish; rampant piracy and crooks on eBay hinder that.
I don't feel comfortable sending you my serial number, especially after having gotten ripped off with a pirated number on eBay... is there any other way to validate the plugins?

Yes.

But first let me point out that since a Lightroom plugin has as much access to your system as Lightroom itself, in having installed my plugins you're already trusting me with every file on your system, so trusting me with your Lightroom serial number seems a relatively small thing.

However, I don't actually need the Lightroom serial number, but rather, the MD5 hash of the serial number (of just the serial-number digits.... no dashes, spaces, newlines, or anything else). It's easy enough to generate on a Mac... send me the result of running
```
     echo -n XXXXXXXXXXXXXXXXXXXXXXXX | md5
```
in a Terminal (the “XXX...” is your serial number without dashes; don't forget the “-n” argument to echo... very important!).

I don't know how to do that on Windows, but there may be a way. There are plenty of sites online that you could use to do the conversion, but then you're trusting some unknown third party with your Lightroom serial number. Up to you.

In any case, I need either that hash or the serial number (from which I'll build the hash myself) to make a validation file for a specific Lightroom install.