U will not be able to fix the cap on your own.
Visit a dentist and he shall fix it on using a suitable cement.
Keep your cap safe and sterile probably in a clean box or packet.
Do not delay the fixation, it might hamper the fit of the cap and also invite other complications like infection, food lodgement, difficulty in chewing etc.
Next Steps
visit a dentist and get it fixed.
Health Tips
Donot delay.
keep the crown safely