What do you mean "disappeared"? Applications do not just disappear - even on the notoriously unstable Windows ME - but need to be uninstalled to disappear.
Can you find Adobe Reader in Add/Remove Programs? If so, can you do a repair install from there?
P.S. If the repair install does not work for some reason, go to C:\Program Files\Adobe... and tell me what version of Acrord32.exe you find there, and I can tell you from where you can download that installer.
You can get old versions of Adobe Reader here :
http://www.oldversion.com/Acrobat-Reader.html (Scroll page down to get to old version list !)
Alternatively you can use an old version of Foxit Reader :
Fiat Lux !