its like it crashes or CPU never crash?

It crashes unless there is a handler for it. Even the handler might decide that it is unrecoverable (e.g. BSOD).
All an exception is is an interrupt.

Look up "triple fault" to see what happens to exceptions that aren't handled.

exceptions are the internal CPU IRQs. exactlly as a peripheral IRQ calls an interrupt vector, the exceptions does the same.

and of course, the handler should exist (at least a IRET), otherwise the CPU crashes.

exception handling depend on the feature you want to do... expand a segment?
load a page?
adjust bound?
do nothing?

ok, lets say there is int 3 instruction.
cpu fires vector 3. Vector 3 has offset > cs limit in idt. Double fault is fired.
Double fault also has offset > limit. CPU enters triple fault. What happens now? How its restarted? Its just an internal reset function that flush registers and cache or there is an acpi function involved?

The CPU will restart from zero by hardware,

Reset hardware lines will be activated to inform the chipset and other buses of this event.

Unfortunately some devices will not recognize this situation correctly and will loose states and configurations.

Anyway you will have to restart from zero with CPU in real mode, BIOS tests, detection of all devices, reinitialization, reloading the OS.. from scratch... very slow.