You need first to do a PING and a TRACERT...
Click your windows START button, then choose RUN
Type "cmd" (no inverted commas) into the Run Box and click OK
You now see a black frame (dos frame) pop up.
You will see something like this: C:\windows\myfiles>_
At the command prompt, type in:-
ping (followed by your website address.... EG:-
**
C:\windows\myfiles>ping https://www.rum21.se**
Hit ENTER
This sends 4 packets of data to the server. If it DOES not find the server, it will tell you. (and Vice Versa).
Then, do a TRACERT (this will show you all the "hops" from your computer to the server, and if there is a breakdown along the way, it will report a TIMEOUT
C:\windows\myfiles>tracert https://www.rum21.se
When you are done, type "exit" at the command prompt and hit ENTER button.
These reports tell you if there is a weak or broken connection from YOUR machine to your server and, depending on the results, you may need to advise your host of the results of the ping and tracert.
The problem is unlikely to be zencart - zen is just software. If you site DOES work when you eventually get to it, then it can't be zencart. It's a server problem, or a network (internet) problem.
If you are on a broadband ADSL modem, disconnect the modem for 10 minutes then start it up again. This may assign you a new IP address (if you are on dynamic IP with your service provider). Sometimes, getting a fresh IP helps resolve network issues.