The crash happened on the A607 at Leadenham near Troops garage about 5.30pm on Monday, January 7 and involved a black BMW car.
The cyclist was taken to Queen’s Medical Centre at Nottingham for treatment.
The road was re-opened at around 11pm.
A Lincolnshire Police spokesperson said: “We would like to hear from anyone who was in the area before or about 5.30pm last evening and saw either the black BMW or the cyclist before the collision or saw the collision itself to contact 101 quoting incident 339 of January 7.
“There are no further details at this stage but we hope to release more later.”
The incident was the second serious collision in the county reported overnight.
Hale Road remains closed and police have been carrying out enquires in the area.
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Lincolnshire Police have found the body of a missing Lincoln boy.
The force appealed at around 4pm on Sunday, April 11, to find Daniel Coupland, 16, from Lincoln, after he went missing.
At 9.30pm police said that a body has been found, which has been identified as the missing boy.
In a statement, Lincolnshire Police said: “We would like to sincerely thank the media and the public for their support with this appeal and request that the family be given time to process this devastating news without intrusion or speculation.”