You do know that there can be multiple 'home screens' right? Just put your finger down on the home screen and slide the whole screen to the left to see the next home screen, you may find your App Store there.
--otherwise...
I have a 3g too and this kind of stuff does happen, first thing I would try is to obviousley "reset" the iphone (just a reboot - you won't lose anything):
How to reset:
Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears.
Failing that, sync your apps with itunes.
Failing that, restore to a backup or sync & save all your personal data/apps then restore, by the way if you don't wanna restore & automatically update to 2.2.1 you can force a version by holding shift & clicking restore in iTunes, then selecting the desired firmware file which can be downloaded first from the list here:
http://www.iclarified.com/entry/index.php?enid =750
Be careful to choose correct one etc. I have used the link for the 3G 2.2.0 (just downloaded a minute ago actually for my second jailbreak) twice now and it's flawless (it's actually directly from apple themselves).
Failing that, it's beyond my experience and you will need to use apples support, either calling, visiting apple store, using knowledge base, using support discussion forums or third party forums, might aswell start here:
http://www.apple.com/support/
Good luck and post back if you need anything.
If you require my help individually, contact me using this link:
http://scr.im/plinth