How do we thaw pipes that froze after our boiler stopped working?
Its a slow process to identify where the pipe or pipes are frozen. 


Turn on faucets and determine which ones work and do not. This will help to identify pipe paths that have an issue. 
With each faucet providing insight, then begin to trace as best you can those plumbing lines. 
The frozen section can have condensation around it, maybe ice or be bulged.  It may also not look any different then a non frozen pipe, which means you need to use the Force, and feel for the section that is frozen. 


Some general areas to start would be exposed pipes near external walls, that are not used that much. Standing water freezes much easier then busy pipes. 

When unfreezing a pipe if you worry you are heating it too fast you probably are.
De-thawing a pipe too fast can cause some very bad results.  So take it slow. 

Turn on the faucet with the issue.
Start with a warm wet towel that you wrap around it.  Never use a blow torch or anything that can cause a dramatic change in temperature.  A hair dryer can work but go slow and do not set it on high and just point and hold.  

Hear is to hoping the frozen section is in an exposed area you can gain access to.