Hi, I've been a contractor since 1971 and a master plumber as well. You should call a professional if at all possible. This can be dangerous work. But if you cannot do so, let's say you live a hundreds miles from civilization and perhaps by yourself then here are simplistic items to check.
To check your electric wtr htr for your symptom: 1) see if you circuit breaker or fuse at your panel box serving the wtr htr has tripped or blown. Reset or replace as needed. 2) reset the hi limit breaker button (usually red)upon the upper thermostat under the cover. Turn off electricity to the unit if you open these covers. This button likely trips due to a relation to another problem symptomizing or beginnig to fail. 3)while that cover is off you should check continuity of the upper element. If that is good then: you may need to replace the upper thermostatt. If it is not good or has no continuity then you need to replace the upper element.
If you think you can tackle this with written advisories then I'll do so if you re-ask here on the site.