You must be assessing your options in a rush here, so let's cut to the chase: If you didn't have any protection measures implemented in the lost phone before the event, like a tracking or remote management app, there's little to nothing you can do to get the item back. However, there are a few things you can do for your own and others' protection that will give you an advantage if this happens again in the future.
Firstly, you need to make sure that the missing phone doesn't cause a data leak, or that it exposes you or your close ones to a scam. Then, we can take care of prevention.
What should I do about a lost phone?
If the missing item is a phone or mobile, you should be able to request an IMEI block to your carrier company, so no one else can insert a SIM card into it and make calls. Blocking phone calls might seem like a secondary issue against the loss of hardware and personal files, but consider that leaving your phone in the wrong hands could leave your contacts exposed to scam calls by people pretending to be you.
To find out what your phone's IMEI number is, dial *#06# in your phone app and keep your IMEI somewhere safe.
Open account sessions could risk personal information
Apps that use a profile to work, such as social media or taxi apps, could contain sensitive data about you. Your first move for safety should be to go to the website of the apps you use, login, and check for open sessions inside your security settings. The same goes for Apple ID, Google Cloud, or any cloud service that your phone Operating System uses.
Can I use IMEI for tracking?
Unfortunately no. Although it sounds promising that there is a unique identifier for a phone and that Telcos are able to record your approximate location based on the cellphone towers that are connecting you to the network, the truth is that location information is not open to the public.
There are plenty of apps out there offering location tracking by IMEI that turn out to be not what they promise. In the end, they will request either your phone number or charge you to do nothing.
How can I track a lost phone in the future?
You can lose a device in many ways, so it's best to be prepared. There are some helpful device tracking apps out there that can make device loss handleable, but unless you have some sort of remote control tool you will need to keep them installed and configured while you have your device nearby, and checked on a regular basis (not daily, but at least every few months. Let's be realistic; you don't really want your life to become dominated by this tragedy). Then, when the time of need comes, you will be able to track it and have a hope of getting it back.
Even though what we really want at that moment is to go back in time and change the events, we can at least save ourselves the trouble in the future and protect our other devices beforehand so we don't have to live worried.