Clubs can become saddled with debt due to financial mismanagement or a poor business model.
If a club is in debt, the Chairman will not sack their current manager nor will they force them to sell any of the club's players. However, the Chairman will take measures to prevent the club going into further debt by not sanctioning the purchase of any new signings.
It is also worth noting that if a club is in debt, then they will not enter into administration nor will they be forced to declare bankruptcy.
If a club is in debt, then it is up to their manager to review their financial management and business model in order to improve the club's finances and clear it's debt.