suddenly a problem occurs on solidwork sketch

Ye part assembly mein banaya hoga. isliye in features mein jo sketch hai unki reference kisi aur part ke edge/face/point kisi bhi entity se attached thi. Ab ye part jab single open kiya to wo reference usko assembly wali mili nahi aur ye X mark aa gya.
Feature tree mein badminton 6 part pe right click kar>>List external references>> Lock References (agar baad mein ye reference use karni hai to) ya fir Break all references (agar baad mein inka koi role nahi hai)

The ? mark represents 'Sketch has gone out of context' Problem. You probably have used sketch reference from an external part e.g. you started the part design in an assembly where you used reference from another part's sketch.

Solution:
a) break the reference by right clicking the sketch and selecting remove external reference or

b) open the problematic sketches and remove all the reference and dimensions and define them again.