What if a timer can not finish all its works before the new cycle time arrives?

Suppose we have a timer which runs every 10 minutes. What if the cycle of its processing takes more than 10 minutes. Does a new thread starts for that? Will it interrupt its current operation? What if a single object is mutated inside the timer?

Sorry if I do not mention any code for that because the problem is clear and also I want to know the complete answer from the viewpoint of a multi-threaded programming geek rather than finding a loose answer by trying to test it via a sample application. Actually, I want to know the logic behind its working mechanism.

If you're using System.Threading.Timer or System.Timers.Timer, the timer will tick again, starting a new thread. See https://stackoverflow.com/a/10442117/56778 for a way to avoid that problem.

If you're using System.Windows.Forms.Timer, then a new tick won't occur until the previous one is finished processing.

Put your code in a Monitor.TryEnter()

object timeCheck= new object();

void Timer()
{
    Monitor.TryEnter(timeCheck) 
    {
        //Code that might take too long 
        //...
        Monitor.Exit();
    }
}

to prevent reentrancy, you might use a static boolean that tells wether the function is allready beeing executed. User a try/Catch/finally and set this boolean to false in the finally to ensure that the boolean does not remain false if you made a mistake in code or if the code failed.
For a faster timer, reentrancy should be prevented by using semaphore (mutex).