Browsers keep a profile folder full of settings, history, extensions and cached data. If that folder is damaged, the browser may crash during start-up before any window appears. Because the damage persists, restarting the computer does not help.
A pending or failed update is another common cause. If the browser was partly updated and now contains a mix of old and new files, it can fail to initialise. Reinstalling over the top usually fixes this without touching your data.
Security software can also block a browser from launching, especially after an update changes the program's signature. The browser looks like a new, unknown application and is quarantined before it can run.
Finally, a single extension set to run at start-up can crash the whole browser. Starting in troubleshooting or safe mode, which disables extensions, proves whether an add-on is the culprit.