+ one for the earlier suggestion to use a web browser to access Microsoft Onedrive on the web to check this. Assuming your files did get properly synced to Onedrive they are extremely unlikely to have been lost. Even if your limit gets set to lower than your file sizes add up to you can still read them, just not write or rewrite files, until you remove some stuff or increase your limit. Use a browser to access Microsoft Onedrive, using a link you know is correct, not a link in an email, so you are sure you really are accessing Onedrive, and see if you can see your files. That should give you some confidence they are safe. Then you can work on getting access back. About the only way to lose them is to delete them all, when you can possibly undelete anyway, or delete the account in which case you will not be able to login.
Advantage with using a cloud based storage is:
1. They are actually very secure, if you use a reputable company.
2. The files are then accesssible from multiple devices, as long as they are syncing properly.
3. Many cloud companies have version control so you can go back to an older version of your file if needed.
4. You can buy what you need.
5. You won't lose your stuff if your computer (phone, tablet, laptop), or your usb pen drive or hard disk, is lost, stolen or damaged.
6. You can share with others.
There are of course disadvantages and you need to decide if you are confident with their security, but in reality if you think that Microsoft Onedrive are going to access your files without your permission then you have to consider they may have remote access to your PC etc. anyway. There is of course a bit more to that but the concept is there. Microsoft Onedrive also has an extra secure system you can use but I am not sure if that costs more.
Remember that if you store stuff on sd card, usb thumb/pen drive, hard disk etc. then you need to encrypt it properly or it is not secure if someone else gets hold of it. Also you will lose it if it is lost, damaged or stolen.
You can use an external hard drive or thumb/pen drive straight into a tablet or phone usb (micro/usb c) port if you use an "on the go" cable, this just sort of reverses the attachment so the phone/tablet can access the drive as long as it has enough power to drive it.