I have a 79 chevy bonanza c 10. The truck will not engage in any gears. The shifter will be in the position but nothing happens when i hit hte gas. Its an automatic transmission. What could the problem be?

The first thing to do is check the trans fluid level and make sure it's full,engine running and truck in park. If it's low top it up and manually shift it slowly through the all the gears. Check the fluid again and top up if necessary. If this cures it you need to be checking for leaks.
If it's full, crawl under the truck and check to make sure the linkage is still hooked together. The shifter linkage could get knocked out of the transmission, it's just a spring loaded fit. With the truck turned off and the wheels blocked so it won't roll, have someone move the shifter through the gears. Watch to see if the shift mechanism on the side of the tranny moves with the shifter. If it doesn't you will have to find where it came apart and fix that.
If this all checks out then it's probable that the trans in going to have to come out and be repaired or replaced because the problem is internal. If it has been slowly getting worse, more gas pedal to keep the same speed or to get it moving, the clutchs are likely worn out. If it happened all of a sudden, it's likely that the pump has gone bad. Either means tearing it apart. You could try taking the pan off of it and checking to see if the filter is plugged. Changing it and the fluid may fix it if the filter is plugged. If you do this, add a bottle of Lucas transmission additive and you may buy yourself some time but a plugged filter and no go means you should start shopping for another transmission. It's just a matter of time before it goes for good. Hope this helps.