wp admin dropdowns not working

Hi, having problems with the admin drop downs, the first i noticed is creating the menu, I can only pick pages to add, if I have a custom link I cannot add as the dropdown will not open to create one, this seems to be happening also when I am on the site health page, I cannot open any dropdowns to see what may be wrong. Though the right sde menu drop downs do work. Should I re-install the core files and if so how would be the safest way, just re-install/overwrite?