You've missed some updates when you were inactive. There have been bunch of changes made to the game. One of them was adding "Forge". This Forge is now used to level up your weapons. When your equipment fully heats up, you can open Forge at Arsenal Stations and level it up using certain number of Fire crystals. You can open Forge by pressing "U" (I think) or by clicking Forge button somewhere at the bottom of the screen. There should be one NPC knight explaining it.

You may want to check some changes that have been made so far. I don't know when you left Spiral Knights, so I'll list here some starting with new UI update:
- new UI
- Dash and Shield Bash (check in the Controls tab with which buttons you can use it)
- Battle Sprites (combat companions) along with few missions focused on them
- free elevators
- New revive system (no more health sharing). You need to use "Spark of Life" (new item, 5* material was renamed to "Storm Chaser") to revive yourself. You can buy them from Supply Depot or get it from treasure boxes, but they are very rare. You can use it to revive fallen party member but that's all. You can't walk to someone and give him half of your HP. If die without Spark of Life, you have to pay 50 Energy for reviving. This doesn't increase with subsequent deaths.
- Emergency revive -> It will revive you for free once per depth. It's that small icon under your health and shield. If you finish a level without using it, it will heal all your missing health
- Removing Mist Energy
- crafting new equipment requires Orbs of Alchemy that can be bought from Supply Depot or found as rare drop from treasure boxes
- leveling up equipment is now done via Forge and requires Fire crystals dropping quite often from treasure boxes
- added 7 more materials dropping from enemies but that's not important
- difficulty settings -> higher difficulty = better drops (Advanced difficulty is normal, Normal is easy. Don't get confused.)
- you can't deposit minerals into gates and they are now randomly generated because people were using Crimsonite, Luminite and Dark Matter for feeding Battle Sprites and were depositing Moonstone and Valestone minerals into gates resulting in more Freeze themes (according to release notes). Now, Moonstone and Valestone are completely useless.
- some missions were slightly changed + new mission "A New Threat"
- Gunner update (giving you guns and gun armor almost for every situation)
- new Vanguard missions
- Spiral Knights has moved from Three Rings (and Sega) under Grey Havens, who are actually a smaller group of people from Three Rings. This has no significant impact on the gamplay itself (apart from the fact, that people are complaining about worse connection, now).

P.S.: Energy prices are way higher than what you've might been used to.