Repairs are being made to a 300-year-old cannon outside Lincoln Castle.
The weapon was found slumped forward on the morning of Monday, December 31 with damage to its wooden base.
However, the cause is not clear. The cannon is part of a pair dating back to 1721. Both have been in Castle Square since 1982.
Lincolnshire County Council said there were no obvious signs of vandalism and the police were not called.
Will Mason, of Lincoln Castle, said: "We found the cannon like this on the morning of December 31.
"It appears that part of the wooden base has broken, but we're not sure what caused this. We're now in the process of getting it repaired."
The cannon has been cordoned off while the work takes place.