You always receive this message when an update is in progress. Like you are updating theme or any plugin. Just wait till update is complete and it will go back to normal and you will be able to access your site easily.
Nope, it just takes 5 secs.
Suggestion is, go to control panel, file manager, theme folder, delete theme from there and upload a new copy, it will be fine. It could have happened because of bad update.