if the tubal was only tied then yes it can be reversed. if they seared it with a laser then it's not reversable.
If you had them tied it is extremely rare to get an ectopic pregnancy. when those happen it is because the tubes have come untied and have come back together and thus opening the passage back up from the ovaries to the the other section of fallopian tube. But this is extremely rare. Very low odds of it happening.
yes, if the procedure fails or if your tubes re-canalize, but chances are very low. it can be undone but chances of a reversal & a successful pregnancy depend on many variables.