Will not start but cranks over

try a security system reset. not sure it will work on yours as I think you are out of US. See link.
https://www.2carpros.com/articles/how-to-reset-a-security-system

Hi! My car is quite an old model Nissan Micra - year 2000 and does not have an alarm or immobilizer as far as I know. Just bought the car and was driving okay for a few weeks. So any other suggestions? By the way great website, Very helpful. Much appreciate your help!

most cars have had security systems since about 1997 so. at least try what it says on link I sent.

It has a key with a special chip in it anti theft device. Is that what you mean?

yes, if the chip is worn out then it will not start. you will probably have to have a mechanic scan it with a professional scanner to find the problem.

I could get another key cut?

you could try, but it has to have the same chip in it. it may not work either as i am not sure if those have to be programmed with a good key or not. we do not have any information on that car because it is not in USA.

Okay thanks!