Can it be fixed easily?

There's three potential causes that I can think of:

• A broken flex cable is possibly the most likely cause. You'd need to partially disassemble the lens and check the flex cables for any damage. There are two flex cables you'd need to check. One is connected directly to the AF motor and the other is the focus FPC flex cable which goes a bit further into the lens and is attached to the inside of the lens barrel.
  • Fix: If the flex cable connected to the AF motor is damaged, you'd need to replace the entire AF motor assembly as the flex cable is soldered on (and I'm not sure that you'd be able to find just the flex cable online). If the focus FPC flex cable is damaged, you're able to buy a replacement and swap them out really easily.
• A bad AF motor could also maybe be the cause. I don't really have troubleshooting steps to determine if the motor is bad, other than just ruling out other causes.
  • Fix: If you believe it is the AF motor that's bad, you'd need to replace the AF motor assembly.
• Damaged or jammed AF gears is the last thing I can think of that could maybe be causing the issue. I'm not sure how likely this cause could be, but if you're already disassembling the lens to check the flex cable, it's worth taking a look at the gears in the AF assembly to see if any are damaged or if there is potentially some debris causing them to jam.
  • Fix: Would need to replace the AF motor assembly if any of the gears are damaged. If a piece of debris is just stuck in the gears, you should be able to use tweezers to get it out.

This guide shows the process to replace the focus FPC flex cable. The guide is for the previous version of this lens, but the interior of your lens should be very similar and it should still be useful to see how to disassemble the lens to allow you to inspect the flex cables and AF assembly. Since there may be some differences, just be sure to keep careful track of where each screw you take out goes.