Windows 10: Dual boot win 7&10, Win 10 boot option is not showing up anymore

I've had both windows in all my computers since the beginning of w10. On this computer win7 was on B drive win10 was on C drive. It is one drive partioned in half Have all my docs, mail, pics, music on external drive. I was using them on both os. Well all of a sudden the page where you chose os doesn't come up anymore and it boots to win7. Windows 10 is still in C Drive, but no way to start it Do I have to start over again or just reload win10? I want to f disk c drive and clean it up, reparation the drive again and start over. Am I going to have a problem loading win 10 or will Microsoft remember my computer?
Thanks in advance.

:)

 

System must start twice to run

Thanks for the reply. The dual boot window at start-up offers Win 7 and Ubuntu (which was uninstalled more than a year ago). The Boot tab in MSCONFIG shows only Win 7. The About Windows tab under tools shows only Win 10. Any suggestions?

 

Can we try out Windows 10 WITHOUT OBLIGATION?

1. How do you set up a dual boot to keep Windows 7 and install Windows 10? Will all my installed programs, games, saved documents, & etc. be saved in both operating systems without affecting them?

2. If I do set up a dual boot to have both Windows 7 and Windows 10 installed to see if I like Windows 10 or not, how do I uninstall Win 10 without affecting Win 7 and everything there (programs, documents, & etc.)?

3. What is the minimum requirements to have both Win 7 and Win 10 installed on one PC and/0r laptop?

 

A screenshot of disk management would help:
Disk Management - How to Post a Screenshot of

Please be sure to widen all the columns so we can see all the data in them.