Solenoid switched by TIP120 transistor stopped working after two weeks
Since you confirmed in comments that the problem was the signal level, I'll explain that a little bit.

You are using an NPN transistor as what is usually called a "low side switch."  Simply put, you are switching the connection between your load (the solenoid) and ground (usually the negative terminal of a powersupply or battery.)

When used this way, an NPN transistor operates as follows:


When current flows through the base (B) terminal of the transistor to the emitter (E) terminal, a much larger current flows through the collector (C) terminal to the emitter.  In this way, a low current signal can drive a high current device.
The connection between the base and the emitter behaves very much like a simple diode.  You have to have more than about 0.7V more on the base than on the emitter.  Since the emitter is grounded, you need 0.7V on the base to make current flow.


Your circuit could never have operated if your control signal was always 2mV.  You've confirmed that the circuit works with a higher voltage, but didn't mention the actual voltage used.



The TIP120 is in fact a darlington  transistor:



As such, the required base voltage is much higher than for a simple transistor.

The datasheet says that you need 2.5V on the base to make the transistor switch on.

To reliably switch your solenoid, you should apply at least 2.5V to the base through a current limiting resistor.

It might be better to use a lower value resistor than the 10k you are using.  The more current through the base, the more current through the collector.

If you get enough current through the collector, it will reach saturation.  The voltage dropped across the collector to emitter will reduce, and you will waste less power in the transistor.  You get more power to the solenoid, and the transistor won't get as hot.



Forgot to mention:

The base voltage value isn't set in stone.  That 0.7V is a typical value that is close enough for most things.  It varies according to how the transistor is made, the temperature, and the current.

The same applies to the 2.5V for your TIP120.  The voltage is somewhere around 2.5V.