my iphone cant detect my sim card

Just to add my two cents to what Shichimi correctly suggested:

Even if not frequent, it could be also a SIM tray (holder) failure.

You can take a look at this support guide too:--> If you see No Service or Searching on your iPhone or iPad - Apple Support

or, if your iphone is a model 7, check if it is eligible for this repair program:

iPhone 7 Repair Program for "No Service" Issues - Apple Support

Regards

Giulio

There are four common reasons for "No Service" to come up.

1. You are in a place where there truly is no signal available. Are other people who subscribe to your service provider able to get signal when standing next to you?
2. Your SIM card is incompatible with your service provider's service offerings. For example if you have a SIM card that only works on 2G and 2.5G, and your service provider is trying to roll out 5G, they may be decommissioning 2G in order to redeploy that spectrum.
3. Your SIM has been deactivated by your carrier. Perhaps they have already prepared a replacement SIM, which you haven't installed yet. Or, your account has been closed by your carrier.
4. Your SIM has died for no particular reason.

You need to diagnose your own situation. If you can, get together with another iPhone user who is also a subscriber to your cellular service provider. Confirm that the other person has good service. Put your SIM in the other person's phone and his SIM in your phone. See whether the problem moves with your SIM or stays with your phone.

Plan the next step from the results of this diagnosis.