The phone's mobile hotspot either will not turn on, or other devices cannot see it or cannot connect to it. The hotspot shares the phone's mobile data, so the fault can be on the phone, on the connecting device, or with the mobile plan itself.
The fix, in order
Check the mobile data first. Turn mobile data on and confirm the phone itself can load a website. A hotspot cannot share a connection the phone does not have.
Toggle the hotspot off and on. Turn it off, wait a few seconds, and turn it back on. Restart the phone if it still will not start.
Check the data limit and plan. Many plans cap hotspot data separately from phone data, or block hotspots entirely. Check the hotspot data allowance and whether the plan includes tethering.
Forget the hotspot network on the connecting device. Remove the saved hotspot network and reconnect, typing the password carefully.
Switch the hotspot band to 2.4 GHz. Some older devices cannot see a 5 GHz hotspot. In the hotspot settings, change the band to 2.4 GHz and try again.
Check the hotspot password and device limit. Make sure the password is correct and the hotspot has not reached its maximum number of connected devices.
Hotspot data can be expensive, and some plans charge extra or slow the connection after a limit. Check the plan before streaming video over a hotspot.
If it still happens
If the hotspot works for one device but not another, the problem is on the connecting device, so forget the network and check its Wi-Fi. If no device can connect, reset the phone's network settings, and check with the carrier whether tethering is enabled on the plan.
Common questions
Why can devices see my hotspot but not connect? The usual causes are a wrong password, the hotspot reaching its device limit, or a band the device cannot use. Forget the network on the device, switch the hotspot to 2.4 GHz, and try again.
Does a mobile hotspot use the same data as the phone? It uses the phone's mobile data, but many carriers count hotspot data separately and cap it at a lower amount. Check the plan to see how much hotspot data is included.