Email coming and going

Hi, yesterday I discovered a friends email had been hacked so I immediately changed my passwords, just in case. Since then I have had nothing but troubles. I am on an iPhone X, an iPad mini, and an iMac and a pc laptop. The pc seems ok. The phone and pad will open mail and show a preview which disappears. If I open mail again it can reappear. Switching off and restart and the mail will have gone again. Any help please?