I have a raspberry Pi 4 (or I should say that my son has one). It worked great for about a week and now it is dead. The red and green lights indicate that the pi is plausibly running but there is no output to any screen available. Please help as he was really enjoying this new life with Pi, but now is quite discouraged that nothing is happening.
There are people willing to help, but "not booting" is unlikely to elicit any helpful response.
You should edit your Question to explain how it was setup and what was subsequently done, what power supply, monitor etc and what other computer you have available.
The following may help diagnose the problem.
Alternatively just performing a fresh installation (use a new SD Card if the old one has anything you want to keep) may help.