If you’ve been in bed for 45 minutes and haven’t been able to fall asleep, you should get up and do something boring/uninteresting until you get tired. If you stay in bed, you’ll associate being in bed with not sleeping, and it’ll just get worse.

If it happens a lot, it’s worth considering CBT-I (cognitive behavioral therapy for insomnia) and getting a sleep study done (to see if you have sleep apnea).