Be sure about the error msg on the screen, There may be either of them-
1. PIN blocked, enter PUK no.
2. SIM card registration fail.
If it is the first one, don't enter any number else your sim will be rejected due to wrong entry of PUK (Max tries-10). And then your error msg will be the second one.
For the first case, you can follow the same as posted above.(Call service operator, i.e-voda, verify few details, name, address- get the code-enter it- change new PIN- sim open)
For the second case you need to visit nearest voda ofc with your ID proof, get sim changed- charges applicable (20 0r 25)