There are lots of advantages about using a cloud based piece of software like Xero but there is one disadvantage. On rare occasions there are issues with the internet which mean you can’t get on.
Most often this will be a problem with your own internet so we’d suggest that you consider having two separate internet connections available if accessing Xero is “mission critical” to you. A 3G hotspot like this can be useful.
On very rare occasions Xero will be inaccessible for reasons at their end. In 5 years of using Xero I’ve experienced two outages when I couldn’t log on (I’m sure there will have been more but I’m a fairly intensive user and it has only affected me twice).
Xero have a status page here which gives regular updates as to what’s going on. If you can’t access Xero then try here first.