how to calculate time complexity of non terminating loops
If the program runs forever, its running time is infinite.  So, if it always enters an infinite loop, its running time is infinite.

This is a degenerate case.  Normally we focus only on algorithms that are certain to terminate (but see footnote).

See also How to come up with the runtime of algorithms? and Is there a system behind the magic of algorithm analysis?.



Footnote: When studying randomized algorithms, this gets relaxed a bit, and we frequently look at algorithms that in principle could run forever if they keep getting an unlucky choice of random bits, but whose expected running time is finite.  However, if you're just starting with algorithms, it's likely that you are dealing with deterministic algorithms, not randomized algorithms, so this is unlikely to come up.