I would think this is a clutch problem. If it won't go into reverse, pump the clutch. If that helps, then your hydraulic clutch system needs to be bled because it has air in the system, causing the clutch not to engage all the way. If possible, peek into the space where the clutch is between the engine and transmission, and see how much material is left on the disc. If it is minimal, then the clutch needs to be replaced. BUT! If you don't fix this problem soon, you will have to replace the trans, because the grinding will wear (and possibly break) the gears inside the transmission.