Very complex and long story, but the short(er) version is that they were taken over by Sisu, who are a hedge fund company, who think you can run a football club by plugging holes and not spending much on the playing squad.
As Red said, there has been a petty dispute with paying the rent for the stadium for over a year. That's because the rent is £1.2m p/a, which is ok for a PL or top end Championship club but for League 1 is a crippling disaster. The club doesn't own the stadium so takes no revenue from match day, concerts, coroporate events - a charity and the local council own 50% each. The only revenue is from tickets and merchandise. The future of the club essentially depends on getting at least 50% ownership or Sisu fucking off.
The club has basically only survived by having a very good youth system and program and selling anyone as soon as a bid comes in. If they could own the stadium there is potential to have a PL club, the size of the city and fan base is well above the likes of Reading, Wigan, QPR, Swansea, Fulham and comparable with Stoke, Norwich, West Brom and Southampton. But it's been a financial nosedive ever since Sisu came in.