I have had a Fender Hot Rod Deluxe for about 1.5 years now and it has recently stopped working. It doesn't produce any sound when playing through either of the two inputs. However, when I plug into the "poweramp in" hole, you can hear the guitar. I am wondering what the cause of this problem may be...I was thinking that the tubes may be dead (havent ever been replaced-- ~3hrs playing time per week). Has anyone had this problem/ does anyone have any suggestions?

Well, its a preamp problem.

But normally when preamp valves go it doesn't result in the sound cutting out completely.

Id say take it to a tech before you change the valves so you don't spend unnecessary money.
All depends on how the preamp tube fails. Had a preamp tube fail in my Mesa amp within a couple of weeks of purchase. The output was totally gone - no sound at all. Replaced the bad tube and it was good as new. Looked at the tube a few days later on my desk and the getter had gone white - the tube was leaky.

As the owner of a tube amp, always have spares available. Tubes are unpredictable and can fail at any time. I know this because I'm an electronics tech. Also, keep spare fuses handy with your amp. You never know when a tube or fuse will blow.