Either the key is not cut properly, or the cylinder is worn out.
First check out if the key isn't worn out. If not, more than likely the tumbler is worn out.
Not knowing what the vehicle is, if you can insert the key and it allows you to turn the mechanism, the key is okay.
Key or ignition tumbler is worn. If you cannot remove the key, then you will have to remove the tumbler assembly and replace it. If you can get the key out, then try a key that is not used very often. If it works, have a duplicate made of that key and destroy the other key with the problems.
it is either the key is worn out or the key cylinder in the ignition is worn out.
bad key switch!
The key or the switch or both could be worn.
No, You do not need a key fob to start a car. When starting a car, you put the key in the ignition and turn. Power comes from car battery, not from the key fob battery!
Worn key or barrel, do you have a new key you can try it with ? If it still doesn't turn properly you got a worn ignition barrel that has to be replaced
Remove the lock, take the cylinder out then replace with the correct bottom pins so they suit the key. Common cause of this fault is when someone does not cut a key properly or is an older key that has worn down over time
the kinetic moecular theory
NO! It is worn out.