Outlook 2003/Exchange

I have an issue I'm having trouble figuring out and need some help with, hopefully someone can. I have a server running W2K Server and Exchange 2000. Now, today I have some clients that are running Outlook 2003 whose inbox folder will not sync with Exchange. Everything was fine until that latter part of last week. Now, if I look on the Exchange server, all their e-mail is there, it's just not being pulled onto their .ost file. Their mail prior to this "failure" is still on their computer, but it's not pulling e-mail from 7/8 and later. All e-mail is stored on the Exchange server. Now I don't typically monitor this (it's not my responsibility) so I don't know what has changed. I know some patches were run last week and there were a few power outages. But I can't say whether or not those have anything to do with this. This is only happening to a few Outlook 2003 clients, not all. Also, I deleted their user profile in hopes that that would solve the problem....it didn't. Please help! Thank you.