Hey. Unfortunately you wont be able to fix it at home. It will come off while eating. Its best you remove the crown and wait till your vaccination to visit the dentist and get a new crown since its a decade old and the reason it came off is because there may be a leakage of saliva and food through the old crown.

Pls store the metal crown with u. Do not leave it back on the tooth as it might dislodge and u can even swallow it. Visit a dentist at the right time get the tooth checked. It has been a decade right? So chances are there might be some wear in the tooth and the fit won’t be accurate warranting the need for a new crown. This time it’s better if u went for a zirconia crown. It’s metal free and has strength of a metal.

Better to remove and keep it aside, as chances of Ingesting it while eating is high as it's not cemented, avoid eating hard food from that side till it's not fixed, there is no temporary home fix that you can do, when you are comfortable visit a dentist

Go and get it fixed Dental cement are not cheap they cost around 2500 For 10 GMs You will not be able to mix it up. So go to dental clinic asap.

Do not put it back on just like that. Fixing crown on root canal treated tooth is a specialized procedure. There is high chances of aspiration i.e it can go and get stuck in ur throat or go inside the windpipe, which can be fatal, if you put the crown back on like that. Instead wait for some days, use a regular mouthwash to keep the concerned tooth clean and visit a dentist as soon as possible. There is a chance of supra- eruption of the tooth if you keep the tooth like that, without the crown, but it can be manged in the dental clinic.