'This Serial Number Appears to be Invalid" -Master Collection CS5

Hello,

I'm having issues with my Master Collection CS5. Here is what happened:

1) I bought a custom built PC, which included MC CS5 in it. Original version, always went online, programs updated automatically with no issues and have been using it for the past 5 years.

2) I recently relocated to another country due to work. As it wasn't feasible to bring my entire PC with me, I just brought along my old hard disk containing my OS & all my other programs/ work documents/ important stuff etc.

3) Got another PC built, but without the HDD. Plugged in my old HDD since I want to use it in my new PC because of all the programs & documents etc. No major issues (repurchased Windows).

4) Tried opening up Photoshop today and message came up saying "Invalid Serial Number" & below that "This Serial Number Appears to be Invalid", which it isn't, so why is it displaying that message? Also, tried to go onto the site to "register an older product" but again that failed.

I would really appreciate help in this as I really need to use this for my freelance job. Thanks.