This problem may go away shortly, but I thought it might be worth mentioning. In some cases I have been moving away from Fast Secure Contact form because it is no longer supported, and has been removed from the WordPress plugin list. It is however still one of the best form managers around. I have been searching for something equivalent.
I found Contact Form 7 which is also very popular, and started to use it. I have found some enhancements so that it can do most of what Fast Secure Contact form did, but it is not a 1:1 replacement.
I found out that while it worked perfectly on IE, Edge, Firefox and Chrome, not all aspects of the form was working for Apple devices if there was an attachment for the form, or the recaptcha function (reduces spam) was present it behaved inconsistently. I made an enquiry with the plugin author and found out there is a problem with WebKit which is used in Apple’s iOS. I expect it will be fixed very shortly, in fact I noticed my iPhone has an update pending right now.
One of the symptoms was that if there was the ability add an attachment you could not send the form if you were using an iPhone or Safari (web browser on Mac devices). In another case I found that after posting a message into the form (where the form subsequently presented some new information in the form area), the screen jumped to the top of page. If the form was not visible within the screen area then you would not see the message.
Do check your forms
I recommend testing your forms at least once a month to make sure they are working. There is no reason why they should stop working, but there have been cases where that has happened due to some other problem in the website. Also check them on a mobile device to make sure that works as well. It is good insurance.
If you find a problem, please let me know immediately. If your site is using an SMTP plugin, then there will be a mailbox associated with anything sent from the website. You can check that to see if there are any bounce back messages in there. These often indicate whether there is a problem or not.