All you've told us is that you have no network. That's literally all we know, so it's very hard to help.
You may have:
* a faulty sim
* a phone that's incompatible with your network
* a faulty phone
* a phone that has been barred (e.g. due to being reported stolen)
* your account with your carrier may have been cancelled, e.g. due to non-payment.
* your network may be suffering an outage and be temporarily unavailable. This can affect some SIMs and not others, depending on the source of the fault.
Any of the above can result in you having no network connection, so without more information we can't really advise.
If your SIM works in another phone then it's not the SIM or the account (though if the SIM is really old it may be incompatible with the new phone). if your phone works with another SIM on the same network then it's not the phone. If others on the same network have also lost signal it's a network fault. The one to check is whether the phone works - if it is OK then the problem is either the sim, the network or your account, and for any of those you need to speak to your service provider. If the problem is you phone then tell us more about the phone (and who your service provider is) and we may be able to suggest something.