If the buttons are not functional the printer is busted. These things happen, sorry.
I have HP LJ cm1415 since 2010 and had to clean the pickup roller once since I bought it. It accepts every type of cheap non-HP toners.
I cannot recommend repairing unless there is a factory service near you that has reasonable prices. A circuit board for 7740 goes for $50 but comes from China. Prices on eBay range from 200 to 400 for used to new 7740 printers.
You might consider one of these printers that can use (eventually) use any type of toner.