maybe this missed / not provided information below could also give some more background on the login problem. I think it could also help to know what happened before the “then it wasn’t”.
### Logs
#### Web server error log
```
Insert your webserver log here
```
#### ownCloud log (data/owncloud.log)
```
Insert your ownCloud log here
```
Please consider what dmitry and tom42 said, but I’d like to mention that login problems are often related to problems with the PHP session storage, you should be able to see more in your web servers error logs though