Regaining your flow when you've been interrupted or have lost focus can be an almost herculean effort if you haven't planned in advance. There's a hack I recommend regardless of the system to get your momentum back with some quick wins in being productive.
It's all about preparation
Personally I use Todoist to track my tasks but this hack works for almost any system. The hack is a simple one. When reviewing my tasks if I find one that can be accomplished in under five minutes but doesn't need to be done right away, I tag it with @kickstart. I usually have anywhere from 5-10 @kickstart tasks in my lists waiting for me. Where the hack comes in is when I realize I need to get back on track.
Using the hack
Part of regaining momentum comes from a few small successes to act as a positive motivator. By filtering my list for @kickstart I can find a few tasks I can knock off the list right away, have a sense of accomplishment, and get back in the being productive flow so I can move on to bigger and better things.
Being productive is about more than just checking off task boxes. It's about putting yourself in the right state to continue to be productive again and again in a constant and predictable manner.
Pro tip for analog people
If you're not a digital person but rather an analog one, you can duplicate the same type of effect. Create a list in your notebook of @kickstart tasks and then just refer to the list when you need to restart your flow.
Another article you may find helpful is Regaining your momentum.