Hi and welcome to just answer.
Initially, remove your needle, remove all thread on both upper and lower, remove the face plate and get a aerosol lubricant for sewing machine (do not use wd40). Spray it to the needle mechanism and set your machine to the zigzag mode. After few minutes press your pedal using zigzag then set it back to straight.
Then put it back all again and try straight stitch again. Replace your needle with new one.
Hope this information will be helpful.
IMPORTANT: RATING IS REQUIRED!! If you are satisfied to my answer please don't forget to rate,,EXCELLENT OR GOOD rating and BONUS are greatly appreciated...