I am an avast user (v.5.0) and recently a duplicate file deleter was run (with permission) on my computer that deleted all duplicate files in the system. I found that it had deleted duplicates on the desktop and all over the place so I did a system restore to the day before. I finished system restoring and when the computer started up again, I found that Avast was not working (according to the Windows Security Center). I clicked the “turn on” button and pressed continue, run, etc. and a message box came up that said “File system shield provider not found”. I cannot seem to turn Avast on.
This was just discussed earlier, here and elsewhere, see: http://forum.avast.com/index.php?topic=58973.0
Seems touch-and-go, some users report problems, others not.
Did you try a simple reboot yet? (no offense, but many seem to not consider this)
You might also want to try a repair, go2 Add/Remove, click on Avast! then Change/Remove (or uninstall/change, depending on op system), choose the repair option then follow the steps.
Reboot didn’t solve it…if I reinstall it will I have to purchase it again?
No you won’t have to purchase it again.
You did save the email with the license information and also the license.avastlic file (used to register your avast version) ?
Where should I download it? If I download the trial version is there a place where I can enter the access key from my email? (the first one I got before it stopped working)
just like you did last time you installed it 
How to register avast 5 pro on page 7 - 8
http://files.avast.com/files/documentation/quick-start-guide-pro-en-ww.pdf also see http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=459.