A force close usually means that one of the applications running in the background crashed OR there was not enough free memory available to run whatever you were trying to run.
I'd recommend the following:
1) If you can write down the EXACT message you get. You'll get a dialog saying X caused Force Close. If you write down what X is (if you get multiple messages, write down all) then it'll be very easy to tell.
2) Archive all apps that you are using.
3) If you downloaded some app recently that uses wireless - like an online game or something - then definitely Archive it to see if it caused issues.
4) Connect your Nook to your PC and move over any unused photos etc. to your PC. you can right click on Nook and select 'Properties' to see available free space. Make sure to move enough items to leave lots of free space.
5) If you have an SD card - remove it.
After doing all this restart your Nook. After this you should be fine.
I suspect the main problem is either:
a) some app you downloaded is running in the background and using up too much resources.
b) you have very little free memory space left and you are running a bunch of memory intensive apps one after the other and that's causing issues.
Have you tried going to a B&N store and having them check. If the above doesn't work, you should try this.