Tandem Breaker is in half-off position. Unable to reset
It is tripped

The halfway position is the normal "tripped" position.  

For more about what the breakers are generally, here. 

To reset it, move it all the way to the "off" position, and then back to the "On" position. Don't make a habit of that.  Whatever you're doing to overload the breaker, stop doing that :) 

Adapting from Euro power

You have 240V power on that side of the pond.  Here, we have 240V, however we center-split it (just like UK does on construction sites) giving us two "sides" of 120V. We use that for small appliances.  Unfortunately that includes kitchen appliances. 



Now, let's have a word about US and EU appliances, particularly heat appliances.  In the EU (checks calendar, after Jan 31, yup) and Britain, appliances just use as much energy as they need for their job.  In the US, appliances are limited to 1500 watts, which is not really enough for a lot of jobs by EU standards.  However your breakers have an 1800W (120V x 15A) capacity, so it means you can't run 2 of them at once regardless! 

So you either tripped it from running a Euro appliance or from running 2 things at once on the same half-circuit.  

Two half-circuits

Remember how I said America splits the 240V to make 120V "legs"?  That circuit is a "Multi-wire branch circuit" which uses both "legs" to provide two half-circuits of 120V and 15 amps each.  Some of your receptacles (and possibly microwave) are on one half-circuit, and something else is on the other half-circuit.  If you can figure out which ones (an electrician can easily mark this for you), you can arrange your appliances so you have one big heat appliance on each one, and then you won't trip!