I adjusted my status from F1 to F2 in the US.?
Unless you are a Canadian citizen, you need a valid F2 visa to enter the US into F2 status. The only exception is if you only went to Canada, Mexico, or a Caribbean island for less than 30 days, in which case you can return to the US, even if your visa is expired, or you did Change of Status and did not get a visa in your new status, and be admitted under Automatic Revalidation for the remainder of the previous period of admission of your previous I-94. People on F1/F2 are admitted for D/S, so this basically means that if you travel to Canada for less than 30 days, you can be admitted back into F2 status for D/S, as long as you show your F2 I-94. Since you for F2 by Change of Status, your I-94 is part of the I-797A approval notice. You must bring that in order to be admitted under Automatic Revalidation.