A maglock uses an electromagnet to hold the door shut.
An electromagnet requires an electrical current in order to operate; otherwise, it no longer holds the door shut. If the sole sources of power are the city's electrical grid and a generator, then when power goes away, the door is unlocked.
A maglock has a third source of power, which is fundamental to its operation. A battery.
When power goes away, the default state of the battery in the absence of voltage applied by the city grid/generator is to discharge, sending current to power the electromagnet and hold the door shut. In conjunction with a positive-signal to unlock, the door is secure. The battery on modern home security maglocks lasts 40-60 hours on loss of power.
If you want to "pick" a locked maglock once it's lost power? You have options.
2) Bring your own power supply.
I'm actually surprised some sort of big portable battery or small portable generator isn't a stock piece of gear. Do some hardware finagling to access the tender parts, and you could counter the battery with a voltage of your own. though if you can do that, the next option's probably more practical. You could send the positive signal yourself to unlock the maglock.
3) Cut the battery's connection
If you could access the battery, all you'd have to do is sever the connection from the battery to the electromagnet without killing yourself. Of course, the battery of a maglock is generally well-protected.
4) Short the battery
If you could access any part of the power line for the battery, you also have the option of shorting it out, so that current isn't going through the electromagnet anymore; it's going through the low-resistance path you just created.
Do keep in mind this low-resistance path results in super high currents, and heat generation is proportional to current squared, and when the chemicals in a battery get hot enough, they have a bad habit of exploding.
5) Safety override
A maglock that fails shut is great for security. It's also horrible for safety. The megacorps may not care MUCH, but it's hard to get away with leaving your employees to die horribly, screaming in a fire concurrent with a loss of power. As such, there is almost always an override for egress. This override is usually on the side you WANT to be on, not the one you ARE on, but if you're trying to get the frag out, it's a useful option.
6) Step to the right and smash
As mentioned, a locked maglock door is often incredibly tough. However, while one reinforced wall is cheap, reinforcing every wall in a room to the same degree is expensive. Step to the right and go Kool-aid man and smash through the wall instead of the door.
Does not work well for vaults.
7) Brute force
A low-end modern maglock shuts with about a third of a ton of force. A high end modern maglock is over a ton. We'll say 1200 kg equivalent force (I'm not using Newtons) to make a nice, round number.
Lifting and carrying is in increments of 15kg. 1200kg / 15kg = 80. Let's say for the purposes of opening this maglock, if you're using a crow bar, you're using Strength x 2 instead of Lifting & Carrying. Now, if your team has three strength 10 orcs and trolls all using crowbars on the maglock at the same time, since the crowbars let us start with a baseline of 15kg per point of strength, and we're doubling strength, we can say they exert a baseline force of 900 kg equivalent force. If, all together, they can manage another 20 hits, then they can straight up pry open that maglock through brute force, but it is one monumental feat.
A low-end maglock would be more on the order of 300 kg equivalent force. If you have a Lifting & Carrying of 20, you could do it barehanded without rolling.
