How long should I wait before following up?

Two to three days. Sooner reads as impatience; later and the conversation is cold, so your follow-up feels like a fresh start. One follow-up, never two.

What should a follow-up say?

Something other than your first message. A new angle, light, with no reproach: "Changing subject entirely — best bar in town, go." What never works: "so?", "are you ignoring me?", "fine, never mind".

What does it mean when a match disappears?

That the person unmatched, or deleted their account. It is not a block and it is not necessarily about you: plenty of people clear out their matches in bulk without reading the open conversations.

Can I tell whether my message was read?

On most apps, no, unless you pay. And the information would not give you anything actionable: read-and-ignored and never-opened call for exactly the same behaviour — one follow-up, then move on.

Why she is not replying, and what to do

The six real reasons

They have not come back to the app. The most common and the most invisible. Plenty of accounts are opened in waves, then left alone for weeks.
They have thirty conversations waiting. On these apps, incoming volume is not symmetrical. Your message joined a queue, not an empty inbox.
They liked in bulk. Some profiles like broadly then sort afterwards. The match never meant particular interest.
Your message did not call for a reply. The one cause you can fix.
They reopened your profile and changed their mind. It happens, and it is the signal that the problem is in the profile, not the message.
They met someone. Nothing to learn from it.

The only lever that is yours

Of those six reasons, five are entirely outside you. Spending time analysing them produces nothing — except the unpleasant sense of having been judged, when in most cases nobody judged anything at all.

The sixth is measurable. If you send twenty messages and get no reply at all, that is no longer chance: it is a signal. Put your twenty messages side by side. If they all look alike, all open with a greeting, and never quote anything from the profile in front of you — you have found the cause.

The follow-up, once

A good follow-up recovers a meaningful share of dead conversations. A bad one closes the door for good.

What works: two or three days later, a message on a completely different angle, light, with no reference to your silence or theirs. "Changing subject: are you the type to finish a series in a weekend or stretch it over six months?"

What closes the door: anything that points at the missing reply. "So?", "are you ignoring me?", "ok, message received", a lone question mark. Those turn an oversight into awkwardness, and awkwardness does not get answered.

When the message is no longer the problem

If your messages are good — specific, short, open — and the silence holds over twenty attempts, then the blockage sits one step earlier. Before replying, most people reopen the profile. That is where your photos decide a second time.

And if the problem is upstream of the match itself, the guide on getting no matches is the one to read.