This error may be caused by many factors, interrupted update process may be one of them.
The simplest solution is to flash microSD card with fresh image, this will solve the problem but you will also loose your data from ROSbot.
Harder solution is to debug the issue.
For this approach, first step would be to log in and perform:
Above command will show you system log since boot. Go through it and check if there is any error or message saying what is preventing system from boot. You may also post full output of command here. Further steps will depend on findings in system log.