- Ensure you have a big enough power distributor for all your modules
- Ensure you have power to the engines to boost
- Ensure you are in normal space, you cannot boost in supercruise
Or try to dock and make sure that everything is repaired in case something is damaged and there you can check that you have the correct power distribution and take off to see if that sorts it out. Maybe turn off some of your systems like weapons and shields but not thrusters etc in case you have bought something that draws too much power.
Check your modules, see if you aren't drawing more power than you can (when this happens, modules get deactivated based on their priority, though I never seen this happen with boost/thrusters)
Because smaller items will fit in larger slots, not insane at all. Just because they will fit, doesn't mean that you should. There are edge cases where you would fit smaller items, but as a rule you typically don't.