It may help if you understand how you access your accounts on each device. If you consider your account on, for example, FaceBook (FB), your account with all your posts, photos etc is not held on your device (iPad device perhaps) but on FB’s servers (computer systems). When you put the FB app on your iPad, you have to give FB (at the minimum) an email address and password. These together are the key to open/use your account, they are your credentials. You probably allow your iPad to look after your credentials for you or else have to enter them every time you want to access FB posts. Having the FB app on your iPad is a shortcut to allow you to see what you have stored on FB’s servers, any messages etc. and you can save what you see on your iPad as well. When you open the app on your device, your credentials are automatically exchanged and checked and, if OK, you are able to use the app. Otherwise you can log in to FB’s website (using your credentials) and see the same data.
When you want to use another device (a laptop?), you will need to go through the same process, download the app, use your key (email and password) and you will be given access to the identical information that you have on your iPad because it is your only information held on FB servers.
If you can understand this process it may help you to realise why you can get shut out of your account on one device (iPad) but not another (laptop). Somehow, the key/credentials on the iPad have changed. As long as you are certain that you know your FB email and password (key/credentials) on the laptop, then you can start again on the iPad. Delete the FB app, download it again, add your email and password and you should have access again. (I say should – but you didn’t have access before so you haven’t lost anything.)
If you are locked out because you have been hacked, things will be different. This means that someone else has worked out/stolen your keys and used them to change your credentials on FB’s servers. You will be locked out on all your devices. You will need to talk to FB to see if they can restore your account.
The more complex and unique your passwords are, the less chance you have of being hacked.