You would be wise to read all you can about how your RAID system works until you understand it well. Go to websites for both your mobo and the VIA chipset you have and look for RAID details and manuals. The nice thing about RAID1 is that it maintains two exact and complete copies WHEN it is working normally. But if one of the drives has a problem it usually can continue functioning with only the remaining good drive for a while.
The starting point for you probably, AFTER reading up on the system and its built-in tools, will be to go into the RAID management system (there's usually a prompt offering this choice on the screen during boot-up for 5 to 10 seconds) and see what it says. It normally will show you the status of the whole RAID1 array and, if there is a problem, which drive is faulty. USUALLY only one drive has trouble and the other is OK.
Now, IF that is your situation, one good option will be to Break the Array, which puts you back into two completely separate drives. Of them, one will be perfectly OK and you will boot and run from it. The other will have trouble to fix. The plan is to fix that problem, then go back into the RAID management system and ADD the fixed second drive so that the RAID1 array can be re-constructed.
BEFORE doing anything else, if you've got to the point where you can use the system with a good drive full of data, BACK IT UP! Then VERIFY that your backup is good. If you can, make a second backup. That puts you in a safe place to work on the drives without losing it all.
Now, what sort of trouble could one drive have? You should certainly download from your drive maker's website some good diagnostic utilities to test the drive thoroughly. Those tests can tell you a lot. In fact, if you end up believing that it is faulty and should be replaced under warranty, the first thing the maker's Tech Support guys will do is tell you to run those tests and report the results to them so they can diagnose properly.
The tests may disclose that your drive is just fine, and the trouble must simply be in bad data on it somewhere. In that case you probably will have in the RAID management tools a way to put this disk back into the RAID1 array and replace all its data, good and bad, with a fresh copy of the good data from the other disk. Problem resolved.
The test may disclose a real hardware error that either can be fixed by you with help from Tech Support, or is justification for an RMA and replacement under warranty. When you get the new drive, again you use the RAID management tools to add it back into the RAID1 array and restore it.
Worst case you decide you have to replace the faulty drive yourself. If so, try to get a replacement as similar to the original as possible. Most importantly, get one the SAME SIZE, or just a little larger. If you have mismatched drive sizes, the RAID system will create an array based on the SMALLER of the two drives available. Then use the RAID management software again to rebuild the array.
OK, absolutely worst case is: both drives are faulty. In that case, after breaking the array, you try working on each drive, at least until you can restore the data and get a good backup off the system. From then what you do to repair or replace both drives depends entirely on what it wrong with each.