How can i recover my computer when all i see is the cursor

Hello, i am not a computer expert, i keep having the same problem again and again, it first appeared a yaer ago and i took my computer to someone who changed the hard disk and put in another make from the original. it crashed again about a month ago and they fixed it. i cannot afford to take it to e specialist every month and i really need my computer cause i work through it. please give me a hint on what to do, i just have a black screen and i see the cursor nothing else.