What happens to a liquid nitrogen canister under pressure if left at room temperature? Will it eventually explode? Or forever stay cold?

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There is no such thing as perfectly isolated cryogenic tank. Some heat will always transfer to the contents. However, cryogenic liquid tanks are not completely sealed. They instead have pressure relief valves. The valve opens when the internal pressure exceeds some upper limit, then closes when the pressure drops below some lower limit. This is the mechanism by which cryogenic liquid tanks (including liquid nitrogen tanks) stay cold.

Suppose a full liquid nitrogen tank is abandoned for decades. What happens over time won't be very exciting. A decade later, the tank pressure will be somewhere between the relief valve's upper and lower pressure limits, the temperature will be very close to ambient, and the mass of nitrogen in the tank will be very small. Almost all of the nitrogen will have boiled off and been vented to the atmosphere.

Now suppose the tank is abandoned and the relief valve fails in the closed position. What happens in the ensuing decades will briefly be exciting. Something will fail, catastrophically. It might be the relief valve, it might be some other valve, or it might be the tank itself. The end result will be an empty tank (or perhaps an empty remnant of a tank).

Finally, let's suppose the tank has no valves and is incredibly strong. For lack of better words, it's made of unobtainium. (But not Thermodynamically Isolated Unobtainium©; that stuff is ridiculously expensive.) After decades of abandonment, the contents of the tank will be at ambient temperature and at a ridiculously high pressure. The contents won't be a liquid (nitrogen's critical temperature is -146.9 °C), but it won't quite be a gas, either. The reactions of a highly pressurized supercritical fluid are somewhere in between those of things we call liquids and things we call gases.