iPhone SE 1st gen won't turn on

Litthium ion batteries do not like sitting for long periods of time without being charged, they may be damaged and need replacing.

Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple

When you say that you have "tried everything", did this include having a new battery installed at an authorized Apple service shop and having the phone checked out for proper operation?

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Charge it for an hour. Then force-restart it:

• Press and quickly release VOLUME UP

     then

• Press and quickly release VOLUME DOWN

     then

• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky. If done correctly this will always work unless the phone has a hardware problem.