How to regain the ability to see through my nikon d5100 viewfinder?

For some reason, I can no longer see the image through my nikon d5100 viewfinder while taking pictures. I would rather not use live view and the playback works fine. It worked fine yesterday and I can't seem to figure out if there's a setting I need to adjust.

Whenever I have a question/problem such as yours, I read the user's guide
(aka "the manual").
It is amazing just how much I always learn from that.

If that doesn't help (or if I'm interested in a product I don't own), I check the manufacturer's website.
They have descriptions and specs for their gadgets, often have user forums, and the decent manufacturers all let you download the manuals in electronic form (so losing the book is no longer a problem).
Getting the PDF is also useful for those times where the printed book is rather small and useless - the PDFs often have more detailed information.

99% of the time reading the instructions does the trick.

If the camera is taking pix and you can't see the subject through the viewfinder, it could be that the mirror is stuck. I would remove the lens and look inside the camera body. You should see a mirror on the bottom at a 45 degree angle. If it isn't there, it may be stuck in the up position. I would, very gently, try to get it to flop down. Even if it did, it would probably be a very temporary solution. Lacking camera repair skills, I would start the process of sending it back to Nikon to be repaired. It you have the receipt and it is less than one year, they should repair it for free.

Have you locked up the mirror? Switch the camera on and take off the lens and look in the camera throat. Should be a mirror at 45 degrees. If you can't see it then it is either locked up (see manual to change) or it is broken.

When the D5100 is in live view mode you can't see through the viewfinder… You have to 'flick' the 'Lv' lever to use the viewfinder.

The most difficult part of your question… Is trying to understand why you don't bother with the instruction manual… If I was you i'd chuck the camera away and take up sewing or knitting… I'd suggest baking… I doubt you'd take the time to read the recipe… And that's the most sensible thing to do.

All of your answers are in the manual.

John and Selina must be soooo… Smart! Smarta**!