No reverse. Only 1st and 3rd gear.

Your Silverado is in limp-home mode. Something has happened to cause your transmission control module to send a signal to the car’s central control computer telling the central unit that a major error has occurred. If you have access to an OBD-II scanner, insert the control head into the diagnostic outlet on the left-hand, lower side of the dash, above the kick-panel, and read the results. That will give you an indication of where the failure has occurred.

There are many things that can cause your Silverado to go into limp-home mode. It might be a faulty solenoid (hopefully this is the problem as it is the easiest and least expensive to fix) or it could be a problem with the valve body, possibly a stuck check valve. It might also be a problem with the torque converter or it might be on the output shaft side. It could also be in the gearing or the clutch packs. It is a wide range of possibilities and without seeing the code I have no idea of the location or the real problem.

Suffice it to say, once you get the code you can have the problem fixed and your transmission should be okay. If the transmission is gone totally — I don’t think it is — you are looking at about a $3,200 investment in a rebuilt replacement.