Yes. Your dentist should evaluate it at each cleaning and exam appointment. If it has broken or otherwise become ineffective it absolutely should be repaired or replaced.
Yes. A dental retainer holds teeth where they are. Even if worn 24/7 there will still be some changes in both your teeth and the retainer. Bring it with you to all dental visits to be adjusted, if necessary. The important thing is to wear it as directed. If you don't teeth may shift and re-treatment may be necessary.
Yes. Retainers may become loose after extended wear and should be checked and adjusted periodically.
No. A dental retainer is used to "retain" proper alignment of teeth. If not worn regularly, or teeth were altered with crowns or restorations, then a retainer may not fit well. Then, the retainer can be made again or revised to fit teeth differently. Thus, a dental retainer needs to be revised if the teeth it was designed to retain have changed, in terms of size and alignment.